Some particle beam therapy systems shape an irradiation field through a multileaf collimator so as to irradiate a particle beam in an irradiation shape based on a treatment plan. In such a particle beam therapy system, in the case where during irradiation of a particle beam, the opening shape of the multileaf collimator deviates from the shape specified by a treatment plan, the dose given to a diseased site, which is an irradiation subject, and the distribution of doses differ from the treatment plan; therefore, it is required to rapidly stop the beam irradiation. Accordingly, monitoring, (confirmation) of the leaf positions, which specify the opening shape, and monitoring of the patient position are important functions for realizing the dose set by the treatment plan and need to be of high redundancy and of high multiplicity; thus, a plurality of detection methods are utilized at the same time.
The leaf position detection methods can roughly be divided into a built-in type detection mechanism integrated in each of the leaves or a driving unit corresponding to each leaf and an external (optical) detection method. As the built-in type detection mechanism, for example, there exists a method in which an encoder is provided in a driving motor for driving the leaf and from the number of revolution of the motor, the leaf position is detected. However, in the case of the built-in type detection mechanism, a detecting member needs to be mounted on each leaf or the driving unit corresponding, to each leaf; thus, the complexity of the system is raised. Accordingly, in the case where a plurality of detection methods is redundantly utilized, as one of them, there is adopted, in many cases, an optical (image) type detection method. For example, there has been proposed a particle beam therapy system (for example, refer to Patent Document 1) in which there are provided a built-in type detection mechanism such as an encoder, and there is further provided an optical detection mechanism that takes an image of the opening shape of a leaf by use of a mirror provided in the irradiation field and a camera provided outside the irradiation field and that measures the position of the leaf from the image, so as to monitor the opening shape of the multileaf collimator in a redundant and multiple manner.
Moreover, as another optical detection method, there is proposed a particle beam therapy system (for example, refer to Patent Document 2) in which a screen is provided in the irradiation field and the opening shape projected on the screen is photographed by a camera. Furthermore, there is also proposed a particle beam therapy system (for example, refer to Patent Document 3) in which a collimated laser beam is irradiated onto outer side surface of a leaf and, from the irradiation position, the position of the leaf is detected.